video {
  display: block !important;
  width: 100% !important;
  height: auto !important;
}

<div style="position: relative; width: 100%; max-width: 100%;">
  <video id="maVideo" autoplay muted playsinline style="width: 100%; height: auto;">
    <source src="https://workdrive.zoho.eu/file/ag9sv5d8347f0f9b543408f22a29a03e6a432?directDownload=true" type="video/mp4">
    Votre navigateur ne supporte pas la vidéo HTML5.
  </video>

  <button onclick="activerSon()" style="
    position: absolute;
    bottom: 10px;
    right: 10px;
    background-color: rgba(0, 0, 0, 0.6);
    color: white;
    border: none;
    padding: 10px 15px;
    border-radius: 8px;
    font-size: 16px;
    cursor: pointer;
    z-index: 10;
  ">
    🔊 Activer le son
  </button>
</div>

<script>
function activerSon() {
  const video = document.getElementById('maVideo');
  video.muted = false;
  video.play();
  event.target.style.display = 'none';
}
</script>